Conventionally, various proposals for improving the resolution and realizing a size reduction of an image sensing device in solid-state image sensing elements used in image sensing devices such as a digital camera and the like have been made. As one of such solid-state image sensing element, the structure of a MOS image sensing element capable of acquiring R, G, and B color components from respective pixels at the same time is disclosed in U.S. Pat. No. 5,965,875. This image sensing device will be briefly explained below.
FIG. 11 shows the structure of a solid-state image sensing element disclosed in U.S. Pat. No. 5,965,875, i.e., a photodiode for each pixel, which is formed to have a three-layered structure (triple-well structure). Referring to FIG. 11, reference numeral 100 denotes a p-type silicon substrate; 102, an n-well formed on the silicon substrate 100; 104, a p-well formed on the n-well 102; and 106, an n-type region. Reference numeral 108 denotes a photocurrent sensor, which has an ammeter 110 for detecting a current of a red (R) component, an ammeter 112 for detecting a current of a green (G) component, and an ammeter 114 for detecting a current of a blue (B) component.
As shown in FIG. 11, in the photodiode, three layers of pn-junction diodes are formed in the depth direction of silicon by deeply forming an n-type layer, p-type layer, and n-type layer in the order named, which are diffused in turn from the surface of the p-type silicon substrate. Light components which have entered the diode from the surface side intrude deeper as they have longer wavelengths, and the incoming wavelength and attenuation coefficient exhibit values unique to silicon. Hence, the depths of pn-junctions of the photodiode with the three-layered structure are designed to cover respective wavelength ranges (R, G, B) of visible light, and currents are independently detected from three layers of the photodiode, thus detecting photo signals of different wavelength ranges.
FIG. 12 is a potential graph of the photodiode with the three-layered structure shown in FIG. 11. The abscissa plots the depth, and the ordinate plots the potential. Note that depths A to D correspond to those shown in FIG. 11.
Since light components that have entered the photodiode with the three-layered structure from the surface side can reach deeper positions as they have longer wavelengths, the region between surface O and depth A mainly accumulates electrons produced by light of the B component having a shorter wavelength, the region between surface O and depth B mainly accumulates holes produced by light of the G component having a middle wavelength, and the region between depths A and D mainly accumulates electrons produced by light of the R component having a longer wavelength.
FIGS. 13A and 13B show a read circuit for reading out charges from the photodiode with the three-layered structure shown in FIG. 11. FIG. 13A schematically shows the read circuit, and FIG. 13B is an equivalent circuit diagram of that circuit. This read circuit can read out charges accumulated on the photodiode. FIG. 14 shows another example of a read circuit for reading out charges from the photodiode with the three-layered structure shown in FIG. 11.
The three readout signals then undergo color signal separation by arithmetic processes, thus reproducing an image.
However, in the conventional structure, holes produced in the region between surface O and depth B, i.e., holes produced by light of the B component in a shallow region close to the surface, and holes produced by light of the G component in a slightly deeper region mix, resulting in color mixture.
FIG. 15A shows the simulation results of signal values obtained upon irradiating the photodiode with the three-layered structure shown in FIG. 11 with light. The solid curves indicate outputs directly obtained from an output circuit, and the broken curves indicate signal values obtained by executing color signal separation by arithmetic processes of obtained signals. Note that a DN output indicates an output from the uppermost photodiode layer, a PWL output indicates an output from the middle photodiode layer, and a NWL output indicates an output from the lowermost photodiode layer.
As described above, in the PWL output from the middle photodiode layer, since the B component mixes with the G component, the G component is separated by calculating PWL output+DN output as a G component signal. At this time, since the PWL output corresponds to holes and the DN output corresponds to electrons, PWL output+DN output=the number of PWL holes—the number of DN electrons, i.e., a subtraction is made in practice. However, since arithmetic operations of the two outputs must be made, noise √{square root over (2)} times that of the read circuit is contained in the G component signal. Also, since the DN output contains many noise components produced by a dark current, the noise of the important G component signal gets worse under their influence.
Likewise, since the NWL output cannot directly used as an R component signal due to poor color separation, NWL output+PWL output+DN output is calculated to obtain the R component signal, thus obtaining the spectral characteristics indicated by broken curve R in FIG. 15. However, as a result of this arithmetic operation, the R component signal contains noise √{square root over (3)} times that of the read circuit. In addition, since the DN output contains many noise components produced by a dark current, the noise of the R component signal gets worse under the influence of them.
FIG. 15B shows spectral characteristics obtained by calculating, using the DN, PWL, and NWL outputs via an infrared cut filter, B component signal=DN output=the number of DN electrons, G component signal=PWL output+DN output=|the number of PWL holes—the number of DN electrons|, and R component signal=NWL output+PWL output+DN output=|the number of NWL electrons−the number of PWL holes+the number of DN electrons|, and then adjusting their gains.
As described above, the photodiode with the three-layered structure can detect light components in different wavelength ranges by setting different depths of pn-junctions. However, the wavelength ranges to be photoelectrically converted of three obtained signals largely overlap each other. For example, when the middle photodiode layer is designed to have peak sensitivity around G color (545 nm), this photodiode layer also photoelectrically converts a photo signal near R color (630 nm) and a photo signal near B color (450 nm) at a ratio of several 10%. When signals which considerably suffer color mixture undergo arithmetic processes, color reproducibility deteriorates, and is readily influenced by noise.
Also, the gain of each photodiode, i.e., the voltage change amount of a photodiode produced per unit charge amount is inversely proportional to pn-junction capacitance C of that photodiode. Since the three photodiode layers inevitably have different areas, and the pn-junction capacitance per unit area also depends on the concentration of each diffusion layer, it is difficult to match the capacitances of the three photodiode layers. Therefore, since the three readout photo signals have different gains, they are hard to process in terms of signal arithmetic operations.
Of the three photodiode layers, two photodiode layers which neighbor in the vertical direction are capacitively coupled via a pn-junction. As charges produced by photoelectric conversion are accumulated in each photodiode layer, the capacitance of that photodiode layer changes. For this reason, the potential of a given photodiode layer is also influenced by a charge amount accumulated on another photodiode layer. For this reason, the linearity of each photodiode layer is disturbed, or changes depending on colors.
More specifically, as shown in FIGS. 13A and 13B, neutral regions are present in the n-type region 106, p-well 104, and n-well 102, and capacitances C1, C2, and C3 are present in their junctions. In addition, when the conventional read circuit shown in FIGS. 13A and 13B reads out voltage signals, parasitic capacitances C1′, C2′, and C3′ of the read circuit are added. As a result, voltages to be read out of respective outputs are read out as complicated voltages in which capacitances C1 to C3 and C1′ to C3′, and the DN, PWL, and NWL outputs influence each other. That is, the read gains from respective outputs are different from each other, and each output is influenced by other outputs. More specifically, even if the number of NWL electrons is zero, when DN electrons are produced, the NWL output is not zero, but a value=DN charge/(C1+C1′) is output.
Also, when voltage signals are read out by the read circuit shown in FIG. 14, the DN, PWL, and NWL outputs influence each other due to the presence of capacitances C1 to C3 of the junctions of the n-type region 106, p-well 104, and n-well 102, and parasitic capacitances C1′ to C3′, and read gains are different from each other.
When signal values read out in this way are used, even when arithmetic operations for obtaining broken curves G and R in FIG. 15A are made, high-quality signal values G and R cannot be obtained.